The conventional catalysts used for purifying the exhaust gas of an internal combustion engine for a vehicle cannot always achieve a desired conversion efficiency through various engine running conditions, because the conversion efficiency of the conventional catalyst varies in accordance with the composition, concentration, flow rate and temperature of the exhaust gas which vary the engine running conditions. Particularly as illustrated by catalyst B in FIG. 1, the conversion efficiency of the catalyst steadily deteriorates as it is contaminated with specific ingredients of the exhaust gas and through thermal degradation as the running distance increases. Meanwhile the catalyst particles with their mechanical strength lowered by thermal degradation tend to be crushed and pulverised. As a result not only does the conversion efficiency of the catalyst drop, but also the crushed or pulverized particles clog the passage through which the exhaust gases flow, so as to decrease the output of the engine; and some portion of such particles are blown out of an exhaust pipe together with the exhaust gas. Moreover, as a the result of thermal shrinkage of the catalyst particles the gross volume of total catalyst particles decreases and a space is formed at the upper part of a catalyst container. Through the space exhaust gases pass without catalystic reaction thereby resulting in an imperfect purification.
To be more specific, the alumina support commonly used in the exhaust gas purifying catalyst is usually a formed product of a mixture of various crystalline transition aluminas which have been obtained by heating, dehydrating and calcinating the alumina hydrate. The alumina support is subjected to a wide range of temperature shocks from ambient temperature to the melting temperature of transition aluminas during calcination and during service. Meanwhile, for the purpose of increasing the reaction rate in the catalystic oxidation of unburnt hydrocarbons and carbon monoxide in the exhaust gas and thereby economizing on the volume of catalyst, the reaction should take place at a still higher range of temperature. Thus, the alumina which constitutes the carrier, being subjected to rigorous temperature conditions, makes a prominent crystal growth; develops sintering accompanied by transformation; and leaves much strain in the support, thereby lowering the mechanical strength of the support the conversion efficiency of the catalyst.
Though the exact progressive thermal transformations of the crystalline alumina is yet to be clarified, it is known as illustrated in FIG. 2 that boehmite changes into .gamma.-alumina, the .delta.-alumina into .delta.-alumina in the range 800.degree.-1050.degree. C., the .delta.-alumina into .alpha.-alumina at 1000.degree.-1100.degree. C., and .zeta.-alumina is obtained at 900.degree. C. or thereabout. Namely, .delta.-alumina can be easily obtained by calcinating boehmite at over 800.degree. C. The .delta.-alumina shows practically stable mechanical properties and .alpha.-alumina is stable to heat. It is well-known that when .theta.-alumina is obtained the mechanical strength is remarkably decreased.
One of the methods to prevent the physical deterioration of the transition alumina support of this kind due to transformation is to maintain a stable mechanically coherent state for a long time in a wide range of service temperatures, therefore the composition and calcination procedure of alumina hydrate have been studied. As a practical method normally growing to stabilize micro crystalline structures of transition alumina in the process of transformation has been utilized.
In the meantime a method of stabilization through addition of certain alkali earth metal compounds has been known. The effectiveness of this method is, however, confined to certain aluminas. Addition of alkali earth metal compounds causes the growth of skeleton structure of alumina, prevents transition to high density, and retains the mechanical strength, but it cannot assure the durability of a conversion efficiency.
Thus the above-mentioned conventional method cannot provide any satisfactory exhaust gas purifying catalyst.